KIEV, Ukraine — Ukraine must pay in advance for Russian gas supplies starting next month, Russian President Vladimir Putin said Thursday, raising pressure on the struggling neighbor as Moscow voiced dismay over what it says is Ukraine’s reluctance to implement an international peace plan.

Putin said in a letter to European leaders that Ukraine’s debt for Russian gas supplies has reached $3.5 billion, and because of its refusal to pay Moscow, it will have to switch to pre-paid gas deliveries starting June 1.

Putin first warned of the move in April in a letter to European leaders, whose nations are customers of natural gas giant Gazprom, which is controlled by the Russian state. Ukraine serves as a major conduit for Russian gas supplies to Europe, and pricing disputes have led to shutdowns in the past.

Putin said gas talks involving Russia, Ukraine and the European Union have failed to reach a compromise, while noting Ukraine’s refusal to pay even though it has received a $3.2 billion bailout from the International Monetary Fund.

Ukraine says it would pay if Moscow restores the price discounts canceled after the toppling of pro-Russian President Viktor Yanukovych in February after months of protests.

Russia denounced Yanukovych’s ouster as a coup and quickly sent its troops to take over Ukraine’s Black Sea peninsula of Crimea, which it annexed weeks later.